Common Ion Effect in Chemistry

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Questions and Answers

What happens to the concentration of hydronium ions (H3O+) when a common ion is added to a solution?

  • It fluctuates but trends towards a neutral state.
  • It decreases, causing the solution to become more basic. (correct)
  • It remains unchanged regardless of the added ion.
  • It increases due to increased acidity.

Which of the following statements about the common ion effect is true?

  • It shifts the equilibrium towards products.
  • It has no significant effect on acid-base equilibrium.
  • It increases the solubility of a salt in solution.
  • It leads to an increase in the pH of the solution. (correct)

In the example provided, which ion acts as the common ion when formic acid is added to sodium formate?

  • Sodium ion (Na+)
  • Hydronium ion (H3O+)
  • Oxygen ion (O2-)
  • Formate ion (HCOO-) (correct)

Which principle is used to predict the shift in equilibrium due to the common ion effect?

<p>Le Chatelier's principle (A)</p> Signup and view all the answers

What is the expected effect on pH when formic acid is added to pure water without a common ion?

<p>The pH would decrease, making the solution more acidic. (C)</p> Signup and view all the answers

What effect does the presence of a common ion have on the dissociation of weak acids in solution?

<p>It leads to less dissociation of the acid. (D)</p> Signup and view all the answers

In the context of the common ion effect, what role do spectator ions play?

<p>They have no effect on the pH of the solution. (A)</p> Signup and view all the answers

How does the common ion effect influence the calculation of pH in solutions?

<p>It complicates the pH calculation by introducing additional variables. (D)</p> Signup and view all the answers

The addition of formic acid to a sodium formate solution results in a higher concentration of hydronium ions compared to pure water.

<p>False (B)</p> Signup and view all the answers

Sodium ions (Na+) influence the pH of the solution when formic acid is added to sodium formate.

<p>False (B)</p> Signup and view all the answers

Le Châtelier's principle explains the shift in equilibrium that occurs with the introduction of a common ion.

<p>True (A)</p> Signup and view all the answers

The calculated pH of a solution with a common ion is lower than the pH when formic acid is added to pure water.

<p>False (B)</p> Signup and view all the answers

A solution with an initial concentration of 0.1 molar formate ions will demonstrate the common ion effect when formic acid is added.

<p>True (A)</p> Signup and view all the answers

Match the terms related to the common ion effect with their corresponding definitions:

<p>Common Ion Effect = Shift in equilibrium due to presence of a common ion Le Chatelier's Principle = Predicts the direction of equilibrium shift when a system is disturbed ICE Table = A tool to organize initial, change, and equilibrium concentrations pH Calculation = Formula used to determine the acidity of a solution</p> Signup and view all the answers

Match the acids or bases with their relevant examples in relation to the common ion effect:

<p>Formic Acid = HCOOH Sodium Formate = HCOONa Hydronium Ion = H3O+ Formate Ion = HCOO-</p> Signup and view all the answers

Match the following scenarios with their outcomes regarding pH changes:

<p>Adding formic acid to a solution with sodium formate = Increased pH due to common ion effect Adding a strong acid to pure water = Lowered pH with no common ion effect Adding formic acid to pure water = Lowered pH without common ions present Adding sodium formate to a solution of formic acid = Decreased concentration of hydronium ions</p> Signup and view all the answers

Match the terms with their effects on hydronium ion concentration in a solution:

<p>High initial concentration of a common ion = Decreased hydronium ion concentration Low initial concentration of a common ion = Increased hydronium ion concentration Addition of a weak acid to pure water = Higher hydronium ion concentration Addition of a salt with a common ion = Equilibrium shifts left, affecting hydronium</p> Signup and view all the answers

Match the components involved in the pH calculation with their roles:

<p>Ka expression = Defines the relationship between species in weak acids Equilibrium concentration = Value used to calculate pH Hydronium ion concentration = Directly related to acidity pH formula = pH = -log[H3O+]</p> Signup and view all the answers

Flashcards

Common Ion Effect

The addition of an ion to a solution that already contains that ion.

Impact of Common Ion on Equilibrium

The presence of the common ion shifts the reaction equilibrium towards the reactants.

Effect of Common Ion on pH

The pH of a solution increases due to a decrease in the concentration of hydronium ions.

Formic Acid and Sodium Formate

A solution containing both formic acid (HCOOH) and sodium formate (HCOONa).

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Initial Formate Ion Concentration

The initial concentration of formate ions (HCOO-) is determined by the molarity of the sodium formate solution.

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Calculating the pH

The pH of the solution is calculated using the Ka value of formic acid and the initial concentrations of formic acid and formate ions.

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Common Ion Effect on pH

The presence of the common ion (formate ion) results in a higher pH (more basic) compared to adding formic acid to pure water.

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Le Chatelier's Principle

A system at equilibrium will shift in a direction that relieves stress.

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What is the common ion effect?

The effect that occurs when an ion already present in a solution is added to that solution. This shifts the equilibrium of a reaction in favor of the reactants.

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What is Le Chatelier's principle and what relation it has to the common ion effect?

Le Chatelier's principle states that if a change of condition is applied to a system in equilibrium, the equilibrium will shift in a direction that relieves the stress.

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How does the common ion effect affect pH?

The common ion effect can be used to calculate the pH of a solution. The pH is calculated using the Ka value of the weak acid, the initial concentration of the weak acid, and the initial concentration of the common ion.

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What relevance does the common ion effect have in buffers?

The common ion effect is particularly important in the context of buffers. Buffers are solutions that resist changes in pH.

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What is the significance of the common ion effect?

The common ion effect is a key concept in chemistry. It is important to understand how it works and how it can be used to make calculations.

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How does the common ion effect relate to buffers?

The common ion effect is important in buffers because it helps to maintain a stable pH. The presence of a common ion helps resist changes in pH when small amounts of acid or base are added.

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Why is the initial concentration of the common ion important in the ICE table?

In the ICE table, we include the initial concentration of the common ion to account for its impact on the equilibrium. This is essential for calculating the pH of a solution using the common ion effect.

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What happens when we add sodium formate to a solution of formic acid?

Adding a common ion to a solution containing formic acid will increase the pH. This occurs because the equilibrium shifts towards the reactants, decreasing the concentration of hydronium ions.

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How is the common ion effect related to Le Chatelier's principle?

The common ion effect explains the shift in equilibrium when a common ion is added to a solution. Le Chatelier's Principle states that a system at equilibrium will shift to relieve stress, and the addition of a common ion acts as that stress.

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Study Notes

Common Ion Effect

  • The common ion effect occurs when an acid or base is added to a solution already containing an ion present in the acid or base.
  • This differs from adding an acid or base to pure water, as the initial concentration of the common ion is not zero.
  • The presence of a common ion shifts the reaction equilibrium to the left, decreasing the hydronium ion (H3O+) concentration.
  • This leads to a higher pH (more basic solution).
  • Le Chatelier's Principle explains this shift: adding a common ion on the product side of the equilibrium drives the reaction backward, favoring the formation of reactants.

The Impact of the Common Ion

  • A common ion lowers the hydronium ion (H3O+) concentration and raises the solution's pH.
  • This equilibrium shift, towards reactants, is predicted by Le Chatelier's principle.
  • Adding a common ion stresses the equilibrium, prompting a shift towards reactants, reducing the common ion's concentration.

Example Calculation: Formic Acid and Sodium Formate

  • Formic acid (HCOOH) is added to a sodium formate (HCOONa) solution.
  • Sodium formate completely dissociates, creating formate ions (HCOO-) and sodium ions (Na+). Sodium ions are spectator ions.
  • The initial concentration of formate ions is crucial and is not zero.
  • The equilibrium of the formic acid dissociation reaction shifts to the left, favoring the formation of undissociated formic acid, due to the common ion.
  • This reaction is represented as:
HCOOH (aq) + H2O (l) <=> H3O+ (aq) + HCOO- (aq)
  • The presence of the common ion (formate ion) lowers the hydronium ion concentration and raises the pH compared to adding formic acid to pure water.
  • Calculation, incorporating the initial common ion concentration and Ka value, yields a different pH (e.g., 3.27) compared to the pH of a solution without the common ion (e.g., 2.13).
  • This difference is a direct result of the common ion effect.
  • The initial concentration of the common ion is included in the ICE table.

Predicting pH Changes

  • Le Chatelier's principle predicts the common ion effect: stressing an equilibrium system by adding a common ion results in a shift towards reactants.
  • This results in a reduced hydronium ion concentration and an increased pH.

Conclusion

  • The common ion effect is crucial in acid-base equilibrium scenarios.
  • Its presence significantly impacts solution pH, especially in solutions already containing common ions.
  • The formic acid/sodium formate example demonstrates how the common ion effect influences pH calculations and values.

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